1. Field of the Invention
The present invention relates to marking of products. In particular, the present invention concerns visually detectable markings on paper and paperboard sheets and methods of producing such products.
2. Description of Related Art
Security markings are commonly incorporated into products to indicate their authenticity or origin. Usually, the markings are applied to the packages of the products. The marking can be visible to the naked eye or its detection may require special instrumentation. An example of a traditional visible security marking is the watermark, which comprises an impression pattern made on the surface of paper. Envelopes and packages are provided with seals and tabs to ensure the integrity of the product. Banknotes are often provided with hologram patterns, holographic security threads and similar marks to complicate counterfeiting of the products. These security markings have also been integrated into product packages, such as casings of computer programs, electronic devices and valuables.
Recently, increased attention has been focused on security markings comprising of diffractive and holographic patterns. The appearance of holograms is visually impressive and the reproduction of a hologram is difficult in the sense that it requires special equipment.
Traditionally, markings of the above kind have been attached to packages as separate labels. Thus, holograms have been manufactured using hot embossing techniques on separate plastic-based films or labels, which are laminated or pasted onto the package. Recently, the level of security provided by hologram labels has been impaired by the high number of counterfeit products and holographic labels produced in Asia.
To further complicate the counterfeiting of the marking and of the packaging blank, it has been suggested in the art to incorporate holograms directly onto the surface of the packaging material. In such embodiments, holographic patterns are embossed onto a special holographic receiving layer on the surface of material using embossing rolls. Problems related to embossing of fibre-based products include, for example, sticking of the embossing tool to the surface of the product, clogging of the embossing tool, visually low reflection level, expensive embossing equipment or coating materials of the product, stringent requirements of the embossing conditions, such as embossing temperature and pressure, fading of the holographic element due to smoothening of the holographic layer, numerous pre-processing steps required before embossing, and low productivity of the production lines. In addition, specialized coatings usually require an approval of authorities, such as the Food and Drug Administration (FDA) of the USA or the Bundesinstitut für Risikobewertung (BFR) of Germany, before they can be used in, for example, food or medicine packages. The problems listed are often interrelated, i.e., solving of one may result in aggregation of other.
A number of methods have been developed, which attempt to tackle the above problems. In the following, the art related to embossable holographic markings is shortly reviewed.
WO 03/002330 discloses a method for making holograms onto a paper or plastic film, which has a coating that includes plastic pigment particles to enhance the embossability and printability of the surface. The embossing is applied on the film before it is imprinted.
US 2004/0218238 discloses a method for creating a holographic image on a printed surface by embossing. The printed roll or sheet is coated with a transparent holographic coating. The document suggests the use of a heated rolling panel embosser unit to produce the holograms on roll material. After the roll embossing phase, the base material is cut with a separate die cutter for further use. The document suggests that a special embossing machine is used for embossing the material in sheet form. However, no further teachings concerning such machines are given.
U.S. Pat. No. 5,164,227 discloses a method for decoration of paper or plastic sheets by embossing. The substrate is provided with a thermoplastic material by extrusion or coating. Before embossing, the coating is heated to soften it and to ensure the reproducibility of the pattern of the embossing device onto the sheeting. The primary embossing equipment described in the document comprises an embossing roll, a nip roll and optionally a take-off roll.
US 2003/0227099 discloses another method for creating a rainbow and/or holographic image on a thermoplastic fibrous sheet or polymeric film. In the known method, the diffraction grating is carried out by employing a polymeric film as an embossing shim. The sheet or film to be embossed is heated before embossing/processing.
US 2004/02077892 suggests a method for producing a second hologram to a hot stamping foil, sticker embossing film or paper coated with thermal lacquer and comprising a first hologram. The holographic product can be used as a sticker for attachment to another product. For enhancing the visibility of the hologram, metallization of the surface of the product is suggested.
In US 2004/0247829, proposes embossing a holographic image on a pigment coating layer. Coating layer is preferred, because of its ability to receive the embossing and to bind printing ink.
U.S. Pat. No. 6,749,925 discloses manufacturing macroscopic lens-like structures on a polymeric layer. The lens-like structures exhibit tilt effects, when viewed from different angles.
The prior diffractive security marking and decoration techniques set high requirements either for the surface to be embossed or for the embossing process. No method has been proposed that would be cost-effectively applicable on high-grade product packages as well as on packages of low price daily consumer goods, fast moving consumer goods (FMCG) or food. The application cost of the marking should be relatively small compared to the cost of the product and its package. In addition, the prior embossing techniques necessitate additional processing of the embossed surfaces in order to achieve visually complete and/or mechanically durable decorations.